1. Field of the Invention
The present invention relates to a method and apparatus for the electrolytic treatment of a metal web. Such electrolytic treatment of a metal web is suitable for various purposes, e.g. plating, electrolytic deposition, anodization, graining and cleaning.
2. Description of the Prior Art
In electrolytic treatment to a metal web on a mass production level, high productivity is required. Therefore, the electrolytic treatment to the metal web tends to be performed at rather high current densities. However, electrolytic treatment at such high current densities promotes the generation of bubbles which causes a non-uniform appearance on the surface of the metal web. Oftentimes uneven color shades appear on the surface of the metal web which indicates that the electrolyzed surface is non-uniform. Moreover, the electrolytic treatment at such high current densities requires the application of higher voltages which increases power consumption. Accordingly, it has been very difficult to accomplish high productivity at such high current densities. The problems identified above have been particularly troublesome when broad metal webs are electrolytically treated on both sides thereof. Thus, the elimination of the bubbles and the drawbacks associated therewith has long been desired.
The generation of bubbles inevitably occurs whenever the metal is electrolyzed. The attachment or adsorption of these bubbles to the surface of the metal, and sojourn or slowness of the movement of these bubbles causes abovementioned non-uniformity of electrolytic treatment on the metal surface. Thus, the electrolyte is required to be vigorously and effectively agitated to minimize the bad effects of the bubbles.
Heretofore, the agitation of the electrolyte has been accomplished by circulating it. FIG. 1 is a schematic sectional view of typical electrolyzer showing a prior art for continuously anodizing an aluminum web. Referring to FIG. 1, an aluminum web 1, is fed by rolls 4 into an first electrolyzer 2 filled with an electrolyte 3 through which electric current is supplied. The aluminum web 1 is carried while being immersed in the electrolyte 3. A plurality of anode plates or electrodes 5, connected to a positive electrode of an power source, are disposed so as to be opposed to the aluminum web 1. Therefore, the aluminum web 1 functions as a cathode during electrolysis in the first electrolyzer 2. The aluminum web 1 is then fed into the second electrolyzer 6. The second electrolyzer 6 and the second electrolyzer 2 are separated by partition plates 7. The second electrolyzer 6 is filled with an electrolyte 8. A plurality of cathode plates or electrodes 9 and 9', connected to a negative electrode of the power source, are disposed so as to be opposed to the aluminum web 1. Therefore, the aluminum web 1 functions as an anode in the second electrolyzer 6. The electrolysis results in the oxidation of the surface of the aluminum web 1, forming an oxide film thereon. During the above operation, the electrolytes 3 and 8 are withdrawn from the bath via suction pipes 10 and then returned to the electrolyzers 2 and 6 respectively via feed pipes 11 by means of pumps P. The electrolytes 3 and 8 are thus circulated and agitated thereby.
However, the stirring function by such an electrolyte circulation is limited and is insufficient to remove bubbles generated by electrolysis from the surface of the aluminum web 1. In addition, bubbles tend to stay longer on the bottom surface of the aluminum web 1 than on the upper surface thereof. Moreover, unlike the electrolyte covering the upper surface of the aluminum web 1, the electrolyte beneath the aluminum web 1 has no free surfaces and therefore tends to be stirred less efficiently than the electrolyte above the web 1 which has free uppermost surface. Accordingly, the lower surface of the aluminum web 1 tends to have a non-uniform anodized film deposited thereon, and the lower surface has a lower quality anodized film formed thereon as compared to the film formed on the upper surface.
In an attempt to eliminate the above disadvantage, the circulation speed of the electrolyte has been increased. Alternatively, the aluminum web has been transported through the electrolyzer in a vertical attitude.
However, increasing the circulation speed of the electrolyte requires a pump with a larger capacity which undesirably increases power consumption and requires extra piping and pump space. Moveover, the resulting increase in the flow rate of the electroyte sometimes generates additional bubbles due to cavitation and air caught by the electrolyte dropping down to the electrolyzer from the outlet port of the feed pipe, thus making it very difficult to eliminate the bubbles as desired. Furthermore, since the electrolyte is vigorously agitated only in the vicinity under the outlet port of the feed pipe, the agitation cannot be effective to a desirable extent using this method.
The second alternative is intended to eliminate the bad effects of the bubbles by transporting the aluminum plate vertically through the bath. However, this method is practically disadvantageous when applied to mass production because of technical difficulties in supplying electric power to the metal web and in maintenance of the apparatus required with this alternative method.
A third method, as disclosed in Japanese Patent Publication No. 21840/80 is to dispose electrical insulating partition plates on each side of the aluminum web so that the partition plates are parallel to one another and extend along a length of the aluminum web in the bath, with the partition plates also extending in a direction substantially perpendicular to the surfaces of the aluminum web. Hence, the partition plates define a channel so that the agitation created by the circulation of the electrolyte can be more effectively concentrated in that portion of the bath in the vicinity of the aluminum web. This method is intended to maximize the effect of the stirring caused by the electrolyte circulation by narrowing and limiting the region of circulation. However, the result is substantially the same as that created when a larger pumping system is utilized as proposed in the first method discussed above, and accordingly the partition plate method just described suffers substantially the same drawbacks as those identified for the first method discussed above.